1. Field of the Invention
The present invention relates to floating floors, tile floors and, more particularly, to floating floors assembled from an array of interconnected subunits, each of which includes a ceramic or stone tile bonded to an injection molded plastic substrate, as well as to a method for manufacturing such subunits.
2. Description of the Prior Art
Several attempts have been made in recent years to create floating floors using ceramic tile as the wearing surface. The applicant is aware that, for a short time, a number of U.S. flooring distributors sold boxes of ceramic tile flooring which interlocked to form a floating tile floor. The tiles were mounted on laminate flooring substrates. However, after consumers experienced severe problems related to tile delamination, poor interlocking performance, dirt build-up between adjoined pieces of tile units, and inability to accommodate significant thermal expansion, Costco recalled the product and wrote off as a loss a large inventory of product.
Probably the most successful floating tile floor system developed to date is covered by U.S. Pat. No. 4,930,286 (hereinafter the '286 patent), which is titled MODULAR SPORTS TILE WITH LATERAL ABSORPTION. This patent, in its entirety, is incorporated in this patent application by reference. Developed by Daniel Kotler as a continuous flat athletic floor covering, the floating floor is assembled from interlocking, injection-molded, modular plastic tiles. Each tile includes a plastic support grid made of two sets of mutually-perpendicular intersecting walls. The intersecting wall structure produces an array of interstitial openings that is bounded by a perimetric wall. A support leg, positioned at each wall intersection point, provides structural reinforcement and stress reduction where the walls intersect. A continuous sheet of plastic material, which overlies the support grid, functions as a playing surface. A peripherally-positioned interlock structure, having both male and female components, enables the coupling of any tile to adjacent tiles. The support grid, the support legs, the continuous sheet of plastic material, and the interlock structure are all injection molded as a unitary piece. The interlock structure provides a continuous, uniform displacement gap between adjacent tiles, and also provides a yielding and resilient response to lateral mechanical forces, as well as to forces produced by thermal expansion. The tiles are preferably molded from a tough polymeric thermoplastic compound. It is suggested that low-density polyethylene and polypropylene copolymer are ideally suited for the application, as both polymers are very tough materials having high impact strength, high resistance to corrosive chemicals, and high resistance to abrasion.
As the athletic floor tiles of the '286 patent are preferably injection molded from polyolefin polymer plastics, a detailed description of polyolefin polymers is in order. Polyolefins are generally considered to be the most useful class of synthetic polymers. They are certainly the most widely-used polymers. Also known as polyalkenes, according to nomenclature established by the International Union of Pure and Applied Chemistry (IUPAC), polyolefins are generally prepared using organometallic catalysts, either in solution or supported on a solid surface. In the 1950s, the German chemist Karl Ziegler developed a catalyst for ethylene (ethene is its IUPAC name) polymerization based on a catalyst formed by the reaction of TiCl4 with Al(C2H5)3. Soon thereafter Giulio Natta of Italy made use of this type of catalyst for the polymerization of propylene (propene is its IUPAC name) to produce polymers with highly regular structures. The intimate details of the reactions of these commercial catalytic processes are not entirely understood, but there are strong indications from more easily studied soluble organometallic catalysts that alkenes coordinate to a metal center and then insert into a hydrocarbon chain, producing a longer-chain hydrocarbon attached to the metal center. Repetition of this process leads to extremely-long-chain hydrocarbon polymers. Polyethylene and polypropylene are the most well-known of this genre of polymers, as these plastics are used in consumer items ranging from milk containers and plastic bags to artificial limbs and car bumpers. Polyolefins, the only plastics that are lighter than water, are also break-resistant, non-toxic, and non-contaminating. Polyolefins are relatively inert. In fact, there is no known solvent for polyolefins at room temperature. They easily withstand exposure to nearly all chemicals at room temperature for up to 24 hours, although strong oxidizing agents eventually cause oxidation and embrittlement. Polyolefins are also damaged by long-term exposure to light.
The polymerisation of ethylene results in an essentially straight chain, high molecular weight hydrocarbon. The polyethylenes are classified according to the relative degree of branching (side chain formation) in their molecular structures, which can be controlled with selective catalysts. Aggressive solvents will cause polyethylene to soften and swell, but these effects are normally reversible. Low-density polyethylene (LDPE) has more extensive branching, resulting in a less compact molecular structure. High-density polyethylene (HDPE) has minimal branching, which makes it more rigid and less permeable than LDPE. Linear low-density polyethylene (LLDPE) combines the toughness of low-density polyethylene with the rigidity of high-density polyethylene. Cross-linked high-density polyethylene (XLPE) is a form of high-density polyethylene wherein the individual molecular chains are bonded to each other (using heat, plus chemicals or radiation) to form a three-dimensional polymer of extremely high molecular weight. This structure provides superior stress-crack resistance and somewhat improves the toughness, stiffness and chemical resistance of HDPE. XLPE is a superior material for moulding very large storage tanks. Ultra high molecular weight polyethylene (UHMWPE), also known as high modulus polyethylene (HMPE) or high performance polyethylene (HPPE), is a thermoplastic. It has extremely long chains, with molecular weight numbering in the millions, usually between 2 and 6 million. The longer chain serves to transfer load more effectively to the polymer backbone by strengthening intermolecular interactions. This results in a very tough material, with the highest impact strength of any thermoplastic presently made. It is highly resistant to corrosive chemicals, with exception of oxidizing acids. It has extremely low moisture absorption, very low coefficient of friction, is self lubricating and is highly resistant to abrasion (15 times more resistant to abrasion than carbon steel). Its coefficient of friction is significantly lower than that of nylon and acetal, and is comparable to teflon, but UHMWPE has better abrasion resistance than teflon.
Polypropylene (PP) is structurally similar to polyethylene, but each unit of the chain has a methyl group attached. It is translucent, autoclavable, and slightly more susceptible than polyethylene to strong oxidizing agents. It offers the best stress-crack resistance of the polyolefins. Products made of polypropylene are brittle at 0° C. and may crack or break if dropped from a height of more than several feet.
Polypropylene copolymer (PPCO), which is also known as polyallomer (PA), is an essentially linear copolymer with repeated sequences of ethylene and propylene. It combines some of the advantages of both polymers. PPCO is autoclavable, and offers much of the high-temperature performance of polypropylene. It also provides some of the low-temperature strength and flexibility of polyethylene.